1. Field of the Invention
The present invention relates to a method of forming an electrode in a semiconductor device, and more particularly to a method of forming a fine electrode on a compound semiconductor layer or an element semiconductor layer constituting a semiconductor device by dry etching techniques.
2. Description of the Related Art
When a gate electrode for a very fine field effect transistor is formed, it is required to form the configuration thereof as expected by patterning a metallic film by using dry etching techniques.
As a method of forming the gate electrode, for example, photoresist 2 is applied onto a metallic film 1 as shown in FIG. 1(a), the photoresist 2 is exposed and developed so as to apply patterning to obtain a desired gate electrode size, and then the metallic film 1 exposed from the photoresist 2 is removed by a reactive ion etching method, thereby to form the metallic film 1 into a configuration of a gate electrode 3.
When etching is performed, the pressure of the ambient atmosphere is set lower than 0.1Pa and gas having very heavy molecular weight or atomic weight is used as etching gas, and also, when plasma is generated in etching, etching conditions are set so that the self-bias thereof becomes 200 V to 400 V. It has been known that etching is performed by striking the metallic film with atoms or molecules having very large energy.
As a result, the configuration of the gate electrode 3 becomes fairly sharp as shown in FIG. 1(b), thus making it possible to produce the gate electrode 3 to such an extent that a border line between a surface of a semiconductor layer 4 and the gate electrode 3 is identified clearly.
In this method, however, since atoms or molecules having very large energy strike the surface of the semiconductor layer 4 around the gate electrode 3, damage is introduced into the semiconductor layer 4, thus deteriorating characteristics of the semiconductor element.
Further, there is such a problem that, in case surface state is formed in a semiconductor layer when a gate electrode of a MESFET or other field effect transistor is formed, a source resistance becomes high.
For example, in a High Electron Mobility Transistor (HEMT), the surface state of the semiconductor layer is increased by the damage, and electrons are caught by the state. As a result, two-dimensional electron gas generated at an interface between an electron supply layer and an electron travelling layer under the semiconductor layer is decreased, and good transistor characteristics are unobtainable.
As against the above, it has been proposed to apply etching in which a chemical reaction is dominant using atoms, molecules or radicals having low energy (reference data: Shin-ichi Tachi and Kazunori Tsujimoto IEEE TOKYO SECTION, Denshi, Tokyo No. 30 (1991) pp. 25-29).
Although the semiconductor layer suffers no damage according to this proposal, isotropic etching is applied to the metallic film since a chemical reaction is dominant, and the pattern of the gate electrode 3 becomes narrower than the pattern of the photoresist 2 as shown in FIG. 1(c).